1. Field of the Invention
The Epidermal Growth Factor Receptor (hereinafter the EGF receptor) is known to be a 170,000 dalton glycoprotein (Reference No. 1) present on the surface of a variety of cell types. A gene for this receptor can be obtained by a process described in U.S. Pat. No. 4,933,294 (Reference No. 2) which is incorporated herein for this purpose. Like other known cell surface receptors, it has an extracellular and cytoplasmic (or intracellular) domain wherein the extracellular domain is capable of binding a substance so that the cytoplasmic domain interacts with another cellular molecule. That is, the interaction between the cytoplasmic domain of the EGF receptor with another cellular molecule is a function of the binding by the substance to the extracellular domain of the EGF receptor. The substance that binds the receptor is known as a ligand. Thus, the term "ligand" is limited only by an ability to affect the extra-cellular portion of the receptor so that the cytoplasmic domain is caused to interact with another molecule inside the cell. This interaction is termed "signal transduction".
For example, the signal transducing function of epidermal growth factor to the EGF receptor results in tyrosine phosphorylation and initiates a cascade of events that culminate in cell division essential in the growth and reproduction of the cell (Reference No. 3).
The cascade of events may be initiated by conformational changes of the receptor or receptor-receptor interactions effected by the ligand which modifies the affect of the receptor on cytoplasmic components.
The term ligand does not imply any particular molecular size or other structural or compositional feature. Furthermore, ligands may be natural or non-natural.
In an effort to find better techniques to design drugs, generally substances having low molecular weight, the research of the pharmaceutical industry is focusing on receptor technology and more specifically on ligands which bind to the receptor.
Conversely, the receptor truncates of the present invention are substances which bind ligands that offend an organism by binding the extracellular portion of an EGF receptor. Adsorptive capacities of the truncates may provide a benefit to an organism against detrimental invasion by an offending substance such as a virus or excess epidermal growth factor receptor ligand.
The present invention provides parts of the EGF receptor to which ligands bind for ligands that bind the EGF receptor itself. The parts are a glycoprotein having selected EGF receptor sequences preferably having substantially fewer amino acids than the EGF receptor itself such that the glycoprotein is relative low molecular weight. In other words, the glycoproteins of the present invention bind ligands of the EGF receptor, but are preferably smaller than an EGF receptor and are soluble proteins and as so may bind a natural ligand without transmitting a signal for the growth and reproduction of a cell.
Among the known natural ligands which bind specifically to the EGF receptor and initiate a cellular growth response are: Epidermal Growth Factor, Transforming Growth Factor .alpha. (Reference No. 4) Amphiregulin (Reference No. 5), and Vaccinia Growth Factor (Reference No. 6.)
Thus, the present invention is portions of the extracellular domain of the EGF receptor which are produced using recombinant DNA methods to yield soluble proteins capable of binding the natural ligands.
2. Description of the Prior Art
The extracellular domain of the EGF receptor is described as an external EGF binding domain comprising 621 amino acids (Reference No. 7). On the basis of amino acid sequence analysis, this extracellular region of EGF receptor can be divided into four domains. It has been suggested that the extracellular portion of the EGF receptor which is flanked by two cysteine-rich domains contributes most of the interactions that define ligand-binding specificity of the EGF receptor (Reference No. 8). The four domains are identified as D.sub.1, D.sub.2, D.sub.3, and D.sub.4.
The portion of the EGF receptor defining ligand binding for the receptor is said to be the D.sub.3 domain within the EGF receptor.
The present invention provides an expressed protein which is the D.sub.3 domain. This protein and additional proteins of the present invention define an independent portion of the EGF receptor which is a properly folded protein capable of ligand binding. Such a protein, that is only a portion of the whole EGF receptor, is not previously described.
The present invention may use either an EGF receptor cDNA sequence (Reference No. 1) or the EGF receptor gene (Reference No. 9) and their expression which are obtained and carried out respectively by conventional methods.